The basic objectives of the proposed research are to study renal tubular function by means of in vitro perfusion of isolated segments of nephrons. Many of the issues we propose to examine are those which cannot be studied with conventional micropuncture techniques in the intact kidney. In the first series of studies we will examine the following properties of the thin ascending limb of Henle: 1) quantitate the unidirectional fluxes and ion permeability coefficients for Na22, Cl36 and C14-urea at varying osmotic conditions; 2) measure hydraulic conductivity under varying baseline osmolalities; and 3) determine the saturation kinetics of Cl36 transport. The second series of studies concerns the thick ascening limb of Henle: 1) determine whether K is actively secreted; 2) examine whether aldosterone effects presumed K secretion, 3) study effects of various diuretics on the active Cl pump; and 4) determine effect of saline expanded plasma on the pump. Next series of studies are designed to characterize calcium transport by the proximal tubule. In these studies the effect of thyrocalcitonin, vitamin D and parathyroid hormone will be examined. The fourth series of studies examines the mechanism of net fluid transport out of the proximal tubule by: 1) further characterization of oncotic pressure gradients effects; 2) defining the anatomical pathway of fluid transport by electron microscopy; and 3) examining the effect of carbonic anhydrase inhibition from both sides of the epithelium. The final series of studies are designed to examine the possibility of active sodium and urea transport by the papillary collecting duct and quantification of the permeability properties of this segment in presence and absence of antidiuretic hormone.